In a previous invention1, it has been shown that catalysts with excellent activity and stability properties for the catalytic dehydrogenation of light paraffins may be obtained by deposition of a Group VIII metal, a Group IVA metal and, optionally, a Group IA metal, on a mixed oxide carrier material, Mg(Al)O. The mixed oxide carrier material is characterised by a high surface area (typically 100–300 m2/g) and a high stability towards sintering. In a more recent invention2, it was shown that improved stability of the carrier material is obtained by increasing the M2+/M3+ ratio (>2) and by increasing the calcination temperature (700–1200° C.).
Mixed M2+ (M3+)O materials may be obtained by calcination of a hydrotalcite-like material (HTC) of general formula:M2+aM3+b(OH)c(An−)d*xH2Oat temperatures in the range 350–800° C.3. The transformation to the mixed oxide phase may be (partly) reversible, depending on the elements, which constitute the original HTC material, the preparation method, as well as the calcination conditions3. It has been reported that a M2+ (M3+)O material, that had been calcined at 500° C., regained it's hydrotalcite-like structure after contacting it with watere.g.: 4,5. It has further been reported that when hydrotalcite-like materials are used as ion-exchange materials for cleaning cooling water, such materials have a much higher ion uptake capacity when contacted with an aqueous solution after calcination to the oxide structure, than the corresponding uncalcined materials4,5. The explanation that was suggested is that ions present in the aqueous solution are taken up as constituent components in the structure as it is re-converted to the hydrotalcite-like form4.
In the literature, numerous examples may be found relating to the use of hydrotalcite-like materials as catalysts or catalyst carrierse.g.:3. Due to it's layered structure, hydrotalcite-like materials and the calcined analogs offer a variety of possibilities for insertion of an active metal, or of a promoter compound, as illustrated by the following examples:                The active metal may be introduced as one of the hydrotalcite cation components, followed by partial decomposition of the structure near the catalyst surface, by which the active metal component is released (e.g. Ni—Al—O compounds for use in steam reforming6).        The active metal may be impregnated on a hydrotalcite-based carrier (e.g. Pt on Mg—Al—O used for aromatisation of n-hexane7).        The active metal may be introduced as anions by anion exchange in the interlayer space (e.g. Mo- or V-containing HTC-based catalysts for the oxidative dehydrogenation of hydrocarbons8).        
The impregnation methods that have been published to date, and which are relevant for the present invention, may be illustrated by the following examples:
In9, a Ru/Mg(Al)O catalyst is prepared by dissolving RuCl3 in water and >>throwing into the flask in one go>> a (Mg—Al—O type) hydrotalcite in the hydroxycarbonate or oxide form. The resulting material is centrifuged, vacuum-dried and, optionally, heat-treated at 500° C. in N2. It is then reduced and used for the hydrogenation of aromatic hydrocarbons at 150° C. In10, the invention is extended to involve other platinum group metals, preferably Pd.
In11, Pt and/or Pd salt is dissolved in a solvent (preferably water), followed by spraying it onto a hydrotalcite support, or (preferably) soaking the hydrotalcite support in the solution. It is underlined that the pH of the solution must be at least about 5 or higher (preferably 6–8), in order to avoid damage of the hydrotalcite. After the metal deposition step, the product is heated to 200–400° C. in an inert or oxidising atmosphere, and reduced at about 50–450° C., before using it as a catalyst for CO oxidation in the temperature range (−60)−400° C.
In7, a calcined hydrotalcite, Mg(Al)O, is impregnated with Pt(NH3)4Cl2 in aqueous solution, dried, calcined in air at 380° C. and reduced in H2 at 430° C. prior to using it as an n-hexane aromatisation catalyst at 480° C. A patent corresponding to the said publication covers (in claim 1) <<aromatisation catalysts comprising a Group VIII metal on a hydrotalcite-type support having in its uncalcined form a hydroxycarbonate structure>>12.
In13, Pt/Mg(Al)O catalysts were prepared by various methods and the final materials compared as n-hexane aromatisation catalysts. Prior to impregnation, the hydrotalcite was calcined at 600° C. for 12–15 hrs. XRD analysis showed diffuse peaks corresponding to MgO after calcination. Impregnation was performed by either incipient wetness impregnation of H2PtCl6 from an aqueous solution, leading to re-generation of the HTC structure, by vapour phase impregnation of Pt(acac)2, or by liquid phase impregnation of Pt(acac)2 dispersed in acetone. During impregnation of Pt(acac)2 from vapour phase or from acetone, the MgO structure of the carrier material was maintained. After impregnation, the catalysts were calcined at 350° C./6 h and reduced in flowing H2 at 400° C./2 h. During calcination, the MgO phase was partly regenerated for the catalyst impregnated from aqueous solution. H2 chemisorption measurements showed that vapour impregnation gave twice as high metal dispersion (H/Pt=1.4) compared to liquid impregnation from aqueous or acetone solution (H/Pt=0.5). n-hexane aromatisation experiments showed that the catalyst prepared from an acidic metal precursor in aqueous solution led to a higher selectivity towards cracking than the other catalysts. This effect was explained by a higher acidity of the metal complex impregnated from aqueous phase. Impregnation of the catalysts with an alkali metal (K) led to improved benzene selectivity.
In our group's previous inventions, an organic solvent was used for metal deposition1.2. The choice of an organic solvent was based on two observations:
First, the materials were to be used at elevated temperatures, where the hydrotalcite is known to be transformed to a MgO structure. The high surface area of the calcined carrier material (typically 100–300 m2/g) compared to the hydrotalcite phase (typically <50 m2/g) was believed to give the best dispersion of the active components (compared to restoration of the hydrotalcite during aqueous impregnation).
Second, the Group IVA metal salt, which is preferably tin in the previous invention1, was known to have a higher solubility in organic solvents than in water14. It is known in the art that tin-containing salts may be dissolved in water after the addition of an inorganic acid, or a mixture of an inorganic and an organic acid15. In the course of the present work, it was discovered that Group IVA metal salts may also be dissolved in water acidified with an organic acid alone. It was further discovered that both aqueous and organic solutions containing Pt and Sn turned red upon dissolution of SnCl2, indicating the formation of a Pt-Sn complex.
Due to environmental concerns, organic solvents may not be the optimal choice for catalyst preparation at a commercial plant scale. Based on such concerns, the original aim of the present work was to develop a metal deposition method based on aqueous metal salt solutions. Surprisingly, it was found that an aqueous suspension of the M2+ (M3+)O carrier material, leading to the reformation of the hydrotalcite-like phase during metal addition, leads to catalysts with improved activity and stability properties compared to those obtained by metal addition to the calcined M2+ (M3+)O phase.
Under industrial conditions, catalyst pellets of a certain size must be used in order to reduce the pressure drop through the catalyst bed, especially in fixed-bed reactors. In order to increase the mechanical strength of the catalyst pellets, binder materials are often added before pelletisation. Alumina is often used for this purpose.17 
Catalytic dehydrogenation of hydrocarbons is a well-known and commercially important process (16). The reactions follow the general reaction equation:CnH2n+2=H CnH2n+H2  (i)
Dehydrogenation reactions are strongly endothermic, and the conversion is limited by thermodynamic equilibrium. As an example, the equilibrium conversion for the catalytic dehydrogenation of propane is approx. 62% under the conditions used in the present work. The Gibbs energy of reaction (i) becomes more favorable with increasing temperatures, leading to higher equilibrium conversions. The equilibrium conversion at a given temperature further decreases with increasing pressure, and increases with an increasing chain-length (n). The major byproducts from dehydrogenation reactions are lighter hydrocarbons resulting from cracking reactions, as well as coke. Hydrogenolysis of the reactant, as well as hydrogenation of unsaturated products, may also occur. Coke formation leads to catalyst deactivation through encapsulation of the active site, and coke must be gasified in regeneration cycles. In some cases, steam is added to the process to prevent coke formation. In those cases, COx may be formed from steam reforming of the hydrocarbons.